Segfault in profiler

[ViralBShah]

This code crashes in julia, when running in the profiler, inside openblas. This is only on mac and not on linux.
https://gist.github.com/blakejohnson/5600044

@profile for ct=1:10 run_sim(); end

A detailed discussion of this issue began when the profiler was in Profile.jl: timholy/IProfile.jl#14

[ViralBShah]

Cc: @timholy @blakejohnson

[blakejohnson]

I assume we are just moving this issue here?

[vtjnash]

Copying this from the old thread also:

using Profile
@profile let 
              tic()
              n = 10000;
              m = 100;
              l = 10;
              A = zeros(n,m);
              for i=1:n
                  for j=1:m
                      c = rand(10,10);
                      A[i,j] = c[2,5];
                  end
                  h = find(A[i,:].>0.5);
                  for k=1:l
                      c = fft(A[:,l]);
                      d = min(abs(c));
                  end
              end
              toc()
              end;

GDB analysis appears to indicate the crash happens when a timer fires during the prolog of a function call which has dwarf info. Perhaps all C code would need to be compiled with -fasynchronous-unwind-tables to make that work. Somehow gdb usually can get it right (or at least not crash).

[timholy]

Is it fair to guess that gdb is not using libunwind?

[Keno]

I've ruled out a bug in the libunwind sigreturn implementation by writing a version that suspends/samples the main thread instead. Same problem still occurs unfortunately.

[Keno]

@vtjnash This may explain the off by one mystery (from Apple's libunwind):

    // if the last line of a function is a "throw" the compile sometimes
    // emits no instructions after the call to __cxa_throw.  This means 
    // the return address is actually the start of the next function.
    // To disambiguate this, back up the pc when we know it is a return
    // address.  
    if ( isReturnAddress ) 
        --pc;

[Keno]

Looking at their unwinder, I found this:

uint64_t savedRegisters = registers.getRBP() - 8*savedRegistersOffset;

which to me appears invalid if the base pointer has not been adjusted yet.

[Keno]

Alright, I compiled the latest version of Apple's libunwind, which doesn't segfault, but it also doesn't give any output, so I'm not sure that's better.

[blakejohnson]

@loladiro by latest version of Apple's libunwind, are you referring to this, or to something later?

[Keno]

Yeah, that's what I'm referring to. I actually managed to figure it out. I'll have a patch shortly.